MEMS-based developments for accelerometers can typically be classified into either a capacitive or a tunneling current architecture. In a capacitive MEMS accelerometer, movement of the proof mass moves an associated capacitor plate either closer or further from an opposing capacitor plate. In this fashion, the resulting capacitance for the capacitive accelerometer varies corresponding to the acceleration it experiences. The change is capacitance is inversely proportional to the square of the separation between the capacitor plates.
In contrast to capacitive accelerometers, tunneling accelerometers utilize a tunneling current that varies exponentially with the separation between the tunneling tip and the counter electrode. Thus, tunneling accelerometers typically offer better sensitivity since relatively small acceleration variations produce relatively larger responses in the exponentially-responding tunneling accelerometers as compared to square-power-responding capacitive accelerometers. A common architecture for tunneling accelerometers involves placing the tunneling tip on a cantilever end portion. An opposing proof mass responds to an applied acceleration by moving closer or further away from the tunneling tip. The flexibility of the cantilever is exploited through the application of a bias voltage between the cantilever end and one or more biasing electrodes to flex the cantilever appropriately so that the tunneling tip is within the tunneling range of the counter electrode. Note, however, that since the proof mass moves orthogonally to the cantilever longitudinal axis (i.e, either towards or away from the cantilever), there is always the danger of a sufficiently strong acceleration causing the tunneling tip to contact the counter electrode on the proof mass. Since the tunneling tip dimensions at the tip apex are typically on the order of just a few atoms, such a contact could readily damage the tunneling tip. Thus, stops or other means are required to prevent the contact, which decreases the achievable measurement range. In addition, the sensitivity of conventional tunneling architectures is limited by the single tunneling tip.
Accordingly, there is a need in the art for robust tunneling accelerometers with improved sensitivity.
Embodiments of the present invention and their advantages are best understood by referring to the detailed description that follows. It should be appreciated that like reference numerals are used to identify like elements illustrated in one or more of the figures.